A common case is when one module is built on another. This can be strictly defined by the
use or
require constructs. A catalog of CPAN modules categorized by that should be usefull. You could easilly check if there are some new modules built on the module you know (or you have just found by some other means). This should be quite effective way of searching since new modules add to the older ones - so if you know a module you would use for the job there is much chance that a module based on it does much of the work you planned to do yourself.
Update: Added some argumentation.